In the fields of chemistry, biochemistry, and molecular biology, there is a need to improve capabilities for carrying out large numbers of reactions using small quantities of materials. As a result, there is a significant and growing interest in employing array technologies where the arrays comprise an ever increasing number of distinct features on a relatively small substrate.
Many methods for making arrays of biological materials are currently available. Generally, DNA arrays are fabricated on a solid substrate by deposition of whole DNA oligomers or complementary DNA or by in-situ synthesis of DNA oligomers. Specific methods for fabricating biological arrays are summarized in international patent publication WO 95/35505. This reference discusses the “dot blot” technique in which a vacuum manifold transfers a number of DNA samples from circular wells to a porous membrane. In addition, DNA sequences can also be synthesized by using a photolithographic technique as discussed in U.S. Pat. No. 5,445,934 to Fodor et al., and by using a capillary dispenser tapping technique as discussed in U.S. Pat. No. 5,807,522 to Brown et al. All of these techniques suffer from inherent limitations that reduce the capacity for producing arrays accurately and reliably.
Arrays may be prepared by a variety of methods employed in the printing industry that do not suffer from the aforementioned limitations. U.S. patent application Ser. Nos. 09/150,504 and 09/150,507 describe forming biomolecular arrays by novel methods and automated devices for moving a printhead over a print surface and for depositing the fluid composition at desired locations on the surface. Other devices used to dispense solutions are described in, for example, U.S. Pat. Nos. 5,658,802, 5,338,688, 5,700,637, 5,474,796, 4,877,745 and 5,449,754. In essence, inkjet printing processing as applied to array fabrication involves feeding a fluid composition into a dispensing chamber of an inkjet printhead and providing a stimulus repeatedly to cause the fluid composition to issue from a nozzle or orifice toward a substrate at desired locations, thus forming an array of features on the substrate surface.
Central to the use of array techniques is the need to deposit uniform features and to avoid cross-contamination. Both non-uniformly deposited features and cross contamination can generate misleading data and thereby compromise experimental integrity. Thus, when an inkjet printhead is used with different fluids, the head must be thoroughly cleaned after contact with each fluid. In addition, a problem with inkjet printheads in general is particulate buildup. Particulates may be introduced into an inkjet printhead when particulate-contaminated fluid is fed into printhead through a fill port as described in U.S. Pat. No. 5,777,648 to Scheffelin et al. Because the cross-sectional area of the dispensing orifice of a printhead tends to be smaller than the cross-sectional area of the fill port, it is possible to pass particulates through the fill port that cannot leave the printhead through the dispensing orifice. One way to minimize the introduction of unwanted particulate matter into the printhead is to load fluid into the printhead through the printhead's dispensing orifice. For example, U.S. patent application Ser. Nos. 09/150,504 and 09/150,507 disclose the transfer of a fluid into a printhead through the printhead's dispensing orifice relying on capillary action.
Particulate buildup in the printhead is also problematic due to the repetitive nature of array fabrication. For example, when a biological array containing thousands of features is to be fabricated, the head will have to be loaded hundreds of times. During this process, the dispensing chamber of the head can become clogged with particulate matter. Especially during the wash out process, fluids tend to dry at the printhead nozzle leaving residue that was originally completely solvated or suspended as small non-agglomerated particulates in the fluid composition. When the nozzle becomes clogged with residue, droplets of fluid may fail to be fully ejected or to follow a desired trajectory. Thus, features become non-uniform in size and shape. Furthermore, once particulate matter becomes lodged within the printhead, the particulate matter provides an additional surface on which contaminants may be adsorbed or trapped thereby increasing the chance of cross contamination.
The predominant method of cleaning an inkjet printhead or “deposition device” is to flush a wash fluid through the deposition device after introducing the wash fluid via the fill port and out the dispensing orifice of the dispensing chamber. See, e.g., U.S. Pat. No. 5,589,861 to Shibata. However, the orifice of the dispensing chamber is quite small, and the orifice's smallest dimension is typically in the range of tens of microns. Therefore, the flow rate of the wash fluid is limited by the small size of the orifice in the printhead, and low flow rates limit the effectiveness of cleaning. When flow rate is in the laminar flow regime, as is typical with ordinary flushing methods, the velocity of the wash fluid at a surface where particulate matter adheres is theoretically zero. Flushing may also cause particulate matter left in the reservoir to be transported into the dispensing chamber. In addition, particulate matter may be simply too large to be passed through the dispensing orifice. Once trapped in the printhead, particulate matter may become further embedded in the inner wall of the inkjet printhead as the result of further flushing.
Another method of cleaning an inkjet head is through sonication. Sonication is a generally well known technique in inkjet printing technology. For example, U.S. Pat. No. 5,877,580 to Swierkowski teaches sonication as a part of a method to dispense chemical fluids from a capillary device. In addition, JP08085202, JP10250060, JP10250108, and JP10250110 describe the use of sonication in conjunction with inkjet printing technology.
It is well known in the art that sonication may effectively result in disintegration of particulate matter or dislodging of particulate matter from the inner surface of a printhead. U.S. Pat. No. 5,574,485 to Anderson et al., for example, provides for a method in which a transducer having a cleaning fluid thereon is placed near a nozzle. A meniscus is formed with the cleaning fluid such that the meniscus bridges the gap between the transducer and the nozzle. Energizing the transducer causes ultrasonic cleaning of the portion of the nozzle contacted by the cleaning fluid. In addition, U.S. Pat. No. 5,757,396 to Bruner provides a method for sonicating ink-carrying channels within an inkjet printhead while purging the channels with ink. However, sonication by itself is typically only effective on surfaces in contact with the medium that couples the sonic energy with the surface, usually a liquid. Furthermore, there is no guarantee that sonication will disintegrate particulate matter too large to exit through the printhead orifice.